Improved types of piles for construction on permafrost soils in the north of West Siberia

IMPROVED TYPES OF PILES FOR CONSTRUCTION ON PE~4AFROST SOILS

IN THE NORTH OF WEST SIBERIA

D. I. Fedorovich, Yu. O. Targulyan, V. S. Savel'ev, and V. M. Bashkirov

UDC 624.139:624.154

In the north of West Siberia, owing to extremely unfavorable technicoeconomic and frozen- soil conditions, in foundation construction wide use is universally made of piles consisting of steel pipes from 108 to 325 and less frequently to 425 mm in diameter. Such piles fully meet the requirements for quick construction of the foundations and ensure their reliability for exacting limitations on the allowable absolute and relative settlements in different structure categories.

Permafrost base soils are used there in accordance with Principle I, that is, in the frozen state. The piles operate as friction piles, transmitting the load to the base soils on account of the shearing resistance on the lateral freezing-together surface and only partially, by 10-20%, through the lower end on account of resistance of the frozen soil to normal compression.

The pile bearing capacity is determined with respect to the base soils and the pile material. In the first case the required value is commonly reached by using a significant length and large cross-sectional dimensions of the pile. In the design and construction practice, piles made from steel pipes are of constant cross section and for this reason the strength properties of the pile material are not completely and uniformly used along the pile length. This leads to excessive bearing capacity of the pile, as regards its material, in comparison with its bearing capacity with respect to the soil, that is, to overconsumption of the pile material.

The considerable length of steel piles results in significant volumes of drilling work and of consumption of grout poured into the hole, and, what is more important, to large material consumption for low-alloy steel pipes, which are in extremely short supply (Fig. la, b).

This raises the problem of substantially improving the design of metal piles for the north of West Siberia, especially for regions where heavily icy, saline, and peaty permafrost soils occur widely. At the present time, considerable experience in this field has been acquired in different regions of the North. Instead of circular or square cross sections of the lower part of the piles in their embedment zone, piles with irregularly shaped sections are used. In the embedment zone, the perimeter of the pile and the area of its freezing- together with the soil are increased, the shearing resistance of the soil being partially realized not along the smooth metal surface but through the soil, for significantly better results However, with these methods the lateral surface of the hole is not fully used for transfer of the load to the base soils, which causes the bearing capacity to be underestimated.

The highly efficient drilling technology applied in the north of West Siberia ensures construction of large-diameter holes, and the realization of the entire volume of them permits increasing the maximum possible bearing capacity of the piles by a factor of 1.5-2.0. For this purpose, in accordance.with the instructions of the Guide for Use of the SNiP II-18-76 Norms, it is necessary to increase the strength of the grout which fills the spaces between the hole and the pile, in order that the total shearing resistances along the lateral surface of the pile and along the surface of contact between the grout and the permafrost soils in the hole walls be equal. The pressure under the pile foot in this case is transmitted practically over the entire hole bottom area. The strength properties of the base soils are fully used, and the maximum pile bearing capacity for the given soils is determined by the hole dimensions.

Scientific-Research Institute of Bases and Underground Structures. Fundamentproekt Institute. Translated from Osnovaniya, Fundamenty i Mekhanika Gruntov, No. 5, pp. 5-8, September-October, 1987.

0038-0741/87/2405-0171512.50 © 1988 Plenum Publishing Corporation 171

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Despite the effectiveness of this method of increasing the bearing capacity of a pile, its constant cross section and equal strength along its length are inevitably connected with underutilization of the material of the pile in its middle and lower parts: in the middle it is realized by no more than one half and in the loer part it is realized by no more than 10-15% and even less.

Another irrational aspect is the principle of transmission of the load from a steel pile (of solid construction) to the frozen base soils on account of the shearing resistance forces along the lateral freezing-together surface, since they are lower by a factor of 8-12 than the forces of resistance of the same soil to normal compression. In order to increase the effectiveness of the transmission of the load to the frozen base soils, the lower part

172

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Fig. 2. Combined piles, a) Of solid steel pipe with open lower end; b) reinforced- concrete pile; c) composite wood-metal (or wood-reinforced concrete) pile of constant cross section (bolted connections); d) com- bined wood-metal pile with widened (wooden) lower part; e) combined pile with metal or reinforced upper part and large-diameter reinforced-concrete shell in lower part; f) the same, with latticed lower part made from prefabricated reinforced concrete, i, I') Hole 450 and 650 mm in diameter; 2) sand-clay grout; 3) 426-mm-diameter pipe made from Class 09G2S-6 low-alloy steel; 4) reinforced-concrete pile of 35 × 35 cm sec- tion; 5)sand concrete; 6) upper limit of permafrost soil; 7) lower wooden part of composite pile; 7') lower widened wooden part of combined pile; 8) coupling bolts of upper and lower parts of piles; 9, i0) steel pipe, 219 and 426 mm in diameter, made, respectively, from Class 09G2S-6 low-alloy steel and from Class V St3 carbon steel; Ii) fixing (erection) nails; 12) sand fill; 13) reinforced concrete or steel upper part of pile; 14) reinforced-concrete shell; 15) spatial latticed frame of prefabricated reinforced concrete.

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of the pile is made in the form of a lattice with transverse elements within the limits of the zone of its embedment in the permafrost mass. In this connection, the lateral surface of the constructional element is increased and, the chief thing, through the transverse elements of the latticed part of the pile it becomes possible to transmit most of the load to the frozen soil-grout which fills the hole on account of the normal pressure forces, instead of the freezing-together forces (Fig. ic), whereby the strength properties of the frozen soils are used in an optimum manner. Moreover, the pile load is transmitted through the lateral surface and bottom of the hole to the permafrost base soils.

The pile top, located above the ground surface and in the layer of seasonal thawing and freezing, operates under extreme conditions (maximum longitudinal and transverse forces and bending moments, frost destruction, high moisture content of the soils, its aggressiveness on the metal and the concrete, heave, etc.) and it should have the minimum possible cross section; yet, its material must be stable against the above-mentioned actions.

The pile bottom, located in the permafrost soil where it is not subjected to frost destruction, undergoes relatively smaller force actions, and for this reason its dimensions should be established on the basis of the need for transmitting the load to permafrost soils which are less strong than the pile material. This ipart of the pile should have the maximum possible cross section.

The latticed construction of the lower part of a friction pile does not transmit 10-20% of the load as in the case of a solid pile, but 60-80% of it to the base soils on account of resistance of the frozen soil-grout to normal compression.

Design and analysis of the soil base of such a pile begin with establishment of the hole dimensions for obligatory consideration of the available drilling equipment. The resistance of the permafrost soil along the lateral surface of the hole and on its bottom, for specific composition and design temperatures of the soil, should be sufficient for transmitting the design load from the pile. Subsequently, the dimensions of the lower latticed part of the pile and the grout composition are established for such a strength that the entire load from the pile is transmitted to the lateral surface and bottom of the hole.

Ordinarily, this is easy to attain since the soil grout for filling the holes is as a rule not much stronger than the permafrost clays of the soil base. If, however, it is signif- icantly stronger (for example, in the case of sand-cement) then the bearing capacity of the pile, determined for its surface, is greater than for the hole outline. In this case, the same design load on the grout which fills the hole can be transmitted for smaller dimensions of the through metal part of the pile. However, the overall dimensions of the hole should continue to be the same which have been used previously. Thus, by increasing the grout strength, it is possible to achieve metal savings since a pile with a soil-cement lower part is obtained (Fig. id). The latticed steel part is not carried to the hole bottom, but for erection and quality control convenience over the entire depth only 2-4 bars can be extended. For the total pile length corresponding to the design depth of the hole, the presence of the bars makes it possible to immediately detect insufficient drilling of the hole during the pile installation process.

All the above considerations refer to piles made from pipes of different diameters, as shown in Fig. le, f, g. The piles shown in Fig. la-f have the same bearing capacity in permafrost clays with a water content of up to 0.4 and a mean annual temperature of -1.6°C.

Combined piles made from solid rolled steel sections in the upper part and with a spatial frame or large-diameter pipe in the lower (Fig. ic-g) are rational for use under significant vertical loads. Their upper part is made from Class 09G2S steel pipe or I-beams, and the lower, located in the soil, to which it transmits only the vertical load, is made from low- cost harbon steel. The metal consumption, in comparison with steel piles made from constant- section pipes, is reduced by a factor of 3-5, whereas for the low-alloy steels it is reduced by a factor of 5-7. Instead of using pipes made from low-alloy steels, the upper part of such piles can be made from I-beams or channels with sections developed in the direction of action of the maximum horizontal forces and bending moments.

When combined piles are used, in addition to the considerable decrease in the quantity of metal, the volumes of drilling and of grout poured into the hole are also reduced, and the pile cost is lowered by a factor of 2-3. Such piles are competitive in comparison with standard reinforced concrete piles even in regions with developed transportation systems.

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Another effective direction is the construction of composite piles with different cross sections and of different materials along their length (Fig. 2).

At the present time, composite piles made from different materials but of uniform cross section are being used. Although they have certain advantages [for example, when the steel in the lower part of the pile (Fig. 2a) is replaced by wood, both the metal consumption and the pile dimensions are reduced], they have not found wide application because of their high cost and the considerable labor involved in their construction (Fig. 2c).

When the diameter of a constant-section pile is increased, the frost heave forces increase also, as a result of which it becomes necessary to increase the pile driving depth for reliable anchoring.

These shortcomings are eliminated by using a antiheave joint incombined piles made from steel pipes of different diameters (see Fig. Ig) as well as piles with widened lower parts made from wood or reinforced concrete (Fig. 2d, e, f).

In all these cases the upper part of the piles is made from Class 09G25 low-alloy steel meeting the GOST 19282-73 Norms, and the lower part is made from abundant, low-cost steels, the pipe walls or angle flanges being 5-6 mm thick, in conformity with the GOST 1050-74" Norms, using low-grade reinforced concrete (see Fig. 2e) or wood.

Also promising is a new type of pile whose upper part is made not from steel but from reinforced concrete placed in a ring of asbestos-cement pipe with inserts at the ends. The dense, strong asbestos-cement pipe has a smooth surface, ensures reliable operation of the pile under extreme conditions, and makes it possible to increase the values of the allowable bending moments transmitted by the pile. The embedment, in the concrete, of the connecting inserts is designed for the acting forces in conformity with the SNiP 2.03.01-84 Norms°

Figure 2f shows a combined pile with a latticed lower part made from flat reinforced- concrete elements which are assembled to form a spatial structure. The vertical elements have different cross sections (the upper sections are larger than the lower ones). The horizontal rings are joined to the vertical elements by electric welding. The consumption of reinforced concrete in this design, in comparison with reinforced-concrete pipes, is reduced by a factor of 3-3.5.

Friction piles with latticed spatial lower parts are most effective when it is necessary to trahsmit large vertical loads (800 kN and more) to the permafrost soils. For example, use of such piles in foundations with a cold cellar under a tank results in a saving of 30-40% in comparison with ordinary reinforced-concrete piles~

An antiheave joint of the socket type (widened foot of the upper part) makes it possible to easily change over to use of combined piles with a wooden or reinforced-concrete lower part (see Fig. 2d, e, f). In all cases it is located below the top limit of the permafrost soils at a depth which is determined by analysis for action of the frost heave tangential forces.

In combined piles, the widened foot of the upper part of the pile fully takes the frost heave forces. Since the foot plays the role of anchor of the upper part of the pile, the joint between its upper and lower parts works under compression only and does not call for construction of section-weakening holes for pins, keys or bolts in the wooden part of the pile at the contact place.

In combined wood-metal piles (see Fig. 2d), the diameter of the pipe for the foot widening is determined from the diameter of the wooden lower part. After installing the upper part of the pile on the lower for butt-end support on a transverse steel plate, both parts are con- nected by means of erection nails (pins). This socket-type joint makes it possible to select the optimum cross section of the upper and lower parts of the pile independently of their diameters.

The basic hole diameters for which the high-efficiency drilling machines available in the north of West Siberia have been designed are 0.45-0.65 m. The force actions on the upper part of the pile in the foundations of most buildings and structures in this region correspond to pipe diameters of 159-325 mm. These dimensions are combined in an optimum manner for use of the antiheave joint shown in Figs. i and 2, which is made from a 325-525-mm diameter, 400-600- mm-long pipe section with a steel plate welded to it. The upper part of the pile, whose cross section is smaller by a factor of several orders of magnitude than at the joint of the lower part of the pile, is welded to the above-mentioned steel plate.

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The compressive load along the pile is transmitted from its upper part by the transverse steel plate of the socket to the end of the lower part, for example the wooden part, whose strength in the direction along the fiber is close to the strength of concrete. In this case, the nails which connect the upper part to the lower are necessary only for preventing the latter from falling from the socket into the hole during the sinking process.

When combined piles with socket-type joints are constructed, the metal consumption is reduced by a factor of 2-4 in comparison with steel piles, the hole drilling depth for piles in permafrost soils is reduced by a factor of up to 2, and the volume of grout poured into the hole is reduced by a factor of up to 5.

In comparison, for instance, with traditional wood-metal piles of constant cross section, the use of a combined pile with an antiheave joint makes it possible to use an optimum section for the upper part of the pile, as calculated for the acting loads, to simplify and facilitate the jointing, and to eliminate section-weakening in the zone of contact between its wooden and metal parts thanks to absence of holes for pins, keys, bolts, etc. Moreover, the drilling work volume is reduced by a factor of up to 1.5, and the volume of grout poured into the hole is reduced by a factor of up to 3.

The piles discussed here can be sunk into permafrost soils by drilling-drop, drop, or combined methods, and the soil grout can be poured into the hole before and after the sinking operation. A simple and inexpensive method of sinking such piles consists in thawing the permafrost soil zone over a diameter larger than the maximum section of the lower part of the pile and in sinking it by vibration.

Yor easy erection connection between the upper and lower parts, a combined pile with an antiheave joint (or foot) must be sunk without vibration, by jacking it into a hole filled with grout (drilling-drop method) or in thawed soil (drop method). To simplify the pile sinking operation, the lower end of the wooden or reinforced concrete parts is made pointed. In the drilling-drop sinking method, the lower end can be flat also.

Thus, the proposed improvement of the types of piles reduces to the condition that the material and constructional shapes of their upper and lower parts are brought into conformity with the characteristics of their real operating conditions, with maximum consideration of the local possibilities of the construction organizations.

Optimization of the types of piles by constructing their upper and lower parts from dif- ferent materials and with different cross sections makes it possible to facilitate the storage and delivery of the pile elements from the factories on account of the overall dimensions and weight of the transportee elements, to bring the pile material bearing capacity closer to the soil bearing capacity, as much as possible, and to reduce the consumption of scarce materials by a factor of several orders of magnitude.

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